Oxide thin film transistor and method of fabricating the same

ABSTRACT

A method for fabricating a liquid crystal display (LCD) device include: forming a gate electrode on a substrate; forming a gate insulating layer on the substrate; forming a primary active layer having a tapered portion to a side of a channel region of the primary active layer on the gate insulating layer, and forming source and drain electrodes on the primary active layer; and forming a secondary active layer made of amorphous zinc oxide-based semiconductor on the source and drain electrodes and being in contact with the tapered portion of the primary active layer, wherein the primary active layer is etched at a low selectivity during a wet etching of the source and drain electrodes, to have the tapered portion.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relate to an oxide thin film transistor (TFT) andits fabrication method, and more particularly, to an oxide TFT having abottom gate structure using amorphous zinc oxide-based semiconductor asan active layer, and its fabrication method.

2. Description of the Related Art

As the consumer's interest in information displays is growing and thedemand for portable (mobile) information devices is increasing, researchand commercialization of light and thin flat panel displays (“FPD”),which substitute cathode ray tubes (CRTs), the conventional displaydevices, has increased. Among FPDs, the liquid crystal display (“LCD”)is a device for displaying images by using optical anisotropy of liquidcrystal. LCD devices exhibit excellent resolution, color display andpicture quality, so they are commonly used for notebook computers ordesktop monitors, and the like.

The LCD includes a color filter substrate, an array substrate and aliquid crystal layer formed between the color filter substrate and thearray substrate.

An active matrix (AM) driving method commonly used for the LCD is amethod in which liquid crystal molecules in a pixel part are driven byusing amorphous silicon thin film transistors (a-Si TFTs) as switchingelements.

The structure of a general LCD will now be described in detail withreference to FIG. 1.

FIG. 1 is an exploded perspective view showing a related art LCD device.As shown in FIG. 1, the LCD includes a color filter substrate 5, anarray substrate 10 and a liquid crystal layer 30 formed between thecolor filter substrate and the array substrate 10.

The color filter substrate 5 includes a color filter (C) including aplurality of sub-color filters 7 that implement red, green and bluecolors, a black matrix 6 for dividing the sub-color filters 7 andblocking light transmission through the liquid crystal layer 30, and atransparent common electrode 8 for applying voltage to the liquidcrystal layer 30.

The array substrate 10 includes gate lines 16 and data lines 17 whichare arranged vertically and horizontally to define a plurality of pixelareas (P), TFTs (T), switching elements, formed at respective crossingsof the gate lines 16 and the data lines 17, and pixel electrodes 18formed on the pixel areas (P).

The color filter substrate 5 and the array substrate 10 are attached ina facing manner by a sealant (not shown) formed at an edge of an imagedisplay region to form a liquid crystal panel, and the attachment of thecolor filter substrates 5 and the array substrate 10 is made by anattachment key formed on the color filter substrate 5 or the arraysubstrate 10.

The LCD as described above is light and has low power consumption, assuch, the LCD receives much attention, but the LCD is a light receivingdevice, not a light emission device, having a technical limitation inbrightness, a contrast ratio, a viewing angle, and the like. Thus, a newdisplay device that can overcome such shortcomings is being developed.

An organic light emitting diode (OLED), one of new flat panel displaydevices, is self-emissive, having a good viewing angle and contrastratio compared with the LCD, and because it does not require abacklight, it can be formed to be lighter and thinner. Also, the OLED isadvantageous in terms of power consumption. Besides, the OLED can bedriven with a low DC voltage and has a fast response speed, and inparticular, the OLED is advantageous in terms of a fabrication cost.

Recently, research for an increase of a size of an OLED display deviceis actively ongoing, and in order to achieve such a large-scale OLEDdisplay device, development of a transistor that can secure constantcurrent characteristics as a driving transistor of an OLED to ensure astable operation and durability is required.

An amorphous silicon thin film transistor (TFT) used for theabove-described LCD may be fabricated in a low temperature process, buthas a very small mobility and fails to satisfy a constant current biascondition. Meanwhile, a polycrystalline silicon TFT has a high mobilityand satisfying constant current bias condition but fails to secureuniform characteristics, making it difficult to have a large area andrequiring a high temperature process.

Thus, an oxide semiconductor TFT including an active layer formed withoxide semiconductor, but in this case, if oxide semiconductor is appliedto an existing TFT of a bottom gate structure, the oxide semiconductoris damaged during an etching process of source and drain electrodes,causing degeneration.

FIG. 2 is a sectional view sequentially showing the structure of arelated art oxide TFT.

As shown in FIG. 2, the oxide TFT of a bottom gate structure isconfigured such that a gate electrode 21 and a gate insulating layer 15are formed on a substrate 10, and an active layer 24 made of oxidesemiconductor is formed on the gate insulating layer 15.

Thereafter, source and drain electrodes 22 and 23 are formed on theactive layer 24. In this case, when the source and drain electrodes 22and 23 are deposited and etched, the lower active layer 24 (inparticular, a portion ‘A’) is damaged to cause degeneration, degradingthe reliability of the device.

Namely, metal for the source and drain electrodes is limited tomolybdenum-based metal in consideration of a contact resistance with theoxide semiconductor. In this case, however, when the source and drainelectrodes are formed through a wet etching, the active layer is damageddue to the physical properties of the oxide semiconductor which isvulnerable to an etchant, and even when the source and drain electrodesare formed through a dry etching, the active layer is degenerated due toback-sputtering and oxygen deficiency of the oxide semiconductor.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an oxide thin filmtransistor (TFT) using amorphous zinc oxide-based semiconductor as anactive layer, and its fabrication method.

Another object of the present invention is to provide an oxide TFTcapable of preventing amorphous zinc oxide-based semiconductor frombeing degenerated when source and drain electrodes are etched, and itsfabrication method.

Additional features and advantages of embodiments of the invention willbe set forth in the description which follows, and in part will beapparent from the description, or may be learned by practice ofembodiments of the invention. The objectives and other advantages of theembodiments of the invention will be realized and attained by thestructure particularly pointed out in the written description and claimsthereof as well as the appended drawings.

To achieve these and other advantages and in accordance with the purposeof embodiments of the invention, as embodied and broadly described,there is provided a liquid crystal display (LCD) device including: agate electrode formed on a substrate; a gate insulating layer formed onthe gate electrode; a primary active layer formed on the gate insulatinglayer and having a tapered portion to the side of a channel region;source and drain electrodes formed on the primary active layer; and asecondary active layer formed on the source and drain electrodes andmade of amorphous zinc oxide-based semiconductor, wherein the secondaryactive layer is in contact with the tapered portion of the primaryactive layer to thus prevent disconnection of the secondary active layerdue to a step of the source and drain electrodes.

To achieve these and other advantages and in accordance with the purposeof embodiments of the invention, as embodied and broadly described,there is also provided a method for fabricating a liquid crystal display(LCD) device, including: forming a gate electrode on a substrate;forming a gate insulating layer on the substrate; forming a primaryactive layer having a tapered portion to a side of a channel region ofthe primary active layer on the gate insulating layer, and formingsource and drain electrodes on the primary active layer; and forming asecondary active layer made of amorphous zinc oxide-based semiconductoron the source and drain electrodes and being in contact with the taperedportion of the primary active layer, wherein the primary active layer isetched at a low selectivity during a wet etching of the source and drainelectrodes, to have the tapered portion.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory and areintended to provide further explanation of embodiments as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention and are incorporated in and constitute apart of this specification, illustrate embodiments of the invention andtogether with the description serve to explain the principles of theinvention.

In the drawings:

FIG. 1 is an exploded perspective view schematically showing a relatedart liquid crystal display (LCD) device;

FIG. 2 is a sectional view schematically showing the structure of arelated art oxide thin film transistor (TFT);

FIG. 3 is a sectional view schematically showing the structure of anoxide TFT according to a first exemplary embodiment of the presentinvention;

FIG. 4 is a sectional view schematically showing the structure of anoxide TFT according to a second exemplary embodiment of the presentinvention;

FIGS. 5A and 5C are sectional views sequentially showing the fabricationprocess of the oxide TFT illustrated in FIG. 4;

FIGS. 6A to 6C are sectional views showing a second masking process indetail according to the second exemplary embodiment of the presentinvention illustrated in FIG. 5B;

FIG. 7 is a scanning electron microscope (SEM) photograph showing adisconnection of a semiconductor due to a step of source and drainelectrodes in an oxide TFT including only a secondary active layer;

FIG. 8 is an SEM photograph showing a primary active layer taperedthrough a wet etching of source and drain electrodes in the oxide TFTaccording to an exemplary embodiment of the present invention; and

FIG. 9 is an SEM photograph showing a state that the secondary activelayer is in contact with the tapered primary active layer in the oxideTFT according to an exemplary embodiment of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

An oxide thin film transistor (TFT) and its fabrication method accordingto exemplary embodiments of the present invention will now be describedwith reference to the accompanying drawings.

FIG. 3 is a sectional view schematically showing the structure of anoxide TFT according to a first exemplary embodiment of the presentinvention, in which amorphous zinc oxide-based semiconductor is used asan active layer.

As shown in FIG. 3, the oxide TFT according to the first exemplaryembodiment of the present invention includes a gate electrode 121 formedon a certain substrate 110, a gate insulating layer 115 formed on thegate electrode 121, a primary active layer 124 a formed on the gateinsulating layer 115, source and drain electrodes 122 and 123, and asecondary active layer 124 b made of amorphous zinc oxide-basedsemiconductor and electrically connected with the source and drainelectrodes 122 and 123.

In this case, the oxide TFT according to the first exemplary embodimentof the present invention includes the secondary active layer 124 b for achannel by using the amorphous zinc oxide-based semiconductor, so thehigh mobility and constant current bias condition are satisfied anduniform characteristics are secured so as to be advantageouslyapplicable to a large-scale display device.

The zinc oxide (ZnO) is a material that can implement three qualities ofconductivity, semiconductor quality, and resistance according to thecontent of oxygen. The oxide TFT employing the amorphous zincoxide-based semiconductor material as the secondary active layer 124 bcan be applicable to a large-scale display device including a liquidcrystal display (LCD) device and an organic light emitting displaydevice.

In addition, recently, much interest and activities are concentrated toa transparent electronic circuit, and the oxide TFT employing theamorphous zinc oxide-based semiconductor material as the secondaryactive layer 124 b has a high mobility and can be fabricated at a lowtemperature, which thus can be used for a transparent electroniccircuit.

In particular, the oxide TFT according to the first exemplary embodimentof the present invention includes the secondary active layer 124 b madeof a-IGZO semiconductor containing heavy metal such as indium (In) andgallium (Ga) in the ZnO.

The a-IGZO semiconductor is transparent allowing visible rays to betransmitted therethrough, and the oxide TFT made of the a-IGZOsemiconductor has a mobility of 1˜100 cm²/Vs, exhibiting higher mobilitycharacteristics compared with the amorphous silicon TFT.

In addition, the a-IGZO semiconductor has a wide band gap, with which aUV light emitting diode (LED) having high color purity, a white LED andother components can be fabricated, and because the a-IGZO semiconductorcan be processed at a low temperature, light and flexible products canbe manufactured.

Moreover, the oxide TFT made of the a-IGZO semiconductor exhibitssimilar uniform characteristics as that of the amorphous silicon TFT,having a simple component structure like the amorphous silicon TFT andbeing applicable to a large-scale display.

As for the oxide TFT according to the first exemplary embodiment of thepresent invention having such characteristics, a carrier density of thesecondary active layer 124 b can be adjusted by adjusting an oxygendensity within a reactive gas during sputtering, thus adjusting thedevice characteristics of the TFT.

In this case, the primary active layer 124 a according to the firstexemplary embodiment of the present invention may be made of amorphouszinc oxide-based semiconductor including a-IGZO semiconductor like theupper secondary active layer 124 b. After a primary semiconductor layermade of oxide semiconductor is formed, wet etching is performed on thesource and drain electrodes 122 and 123. Then, the source and drainelectrodes 122 and 123 are etched at a low selectivity, generating atapered portion formed at the side of the channel region. In this case,the tapered portion formed at the side of the primary active layer 124 aacts as a contact region being in contact with the upper secondaryactive layer 124 b, thus preventing a disconnection of the secondaryactive layer 124 b due to a step of the source and drain electrodes 122and 123.

As for the oxide TFT according to the first exemplary embodiment of thepresent invention, after the source and drain electrodes 122 and 123 areformed, a-IGZO oxide semiconductor is deposited to form the secondaryactive layer 124 b for a channel, thereby basically solving the problemof degeneration of the oxide semiconductor generated when the source anddrain electrodes 122 and 123 are etched.

Namely, the oxide semiconductor is exposed to the process such as thedry etching of the source and drain electrodes during the devicefabrication, its characteristics are changed to degrade the device orits uniformity. Thus, in order to solve this problem, in the exemplaryembodiment of the present invention, the source and drain electrodes 122and 123 are formed, on which the structure of the secondary active layer124 b used as a channel is applied. This structure has shortcomings oflow efficiency that the secondary active layer 124 b is disconnected orcontacted due to the a step of the source and drain electrodes 122 and123, but in the exemplary embodiment of the present invention, suchshortcomings are improved such that the primary semiconductor layer madeof oxide semiconductor is formed, and then, the primary active layer 124a is formed to have a tapered portion by wet etching the source anddrain electrodes 122 and 123 to secure a contact area with the secondaryactive layer 124 b. In order to freely apply the etching process of thesource and drain electrodes without a limitation and improveohmic-contact characteristics between the oxide semiconductor and thesource and drain electrodes, the source and drain electrodes may beformed as a dual-layer. This will now be described through a secondaryexemplary embodiment of the present invention.

FIG. 4 is a sectional view schematically showing the structure of anoxide TFT according to a second exemplary embodiment of the presentinvention, which includes the same elements as those of the oxide TFTaccording to the first exemplary embodiment of the present inventionexcept that the source and drain electrodes are configured as adual-layer.

As shown in FIG. 4, the oxide TFT according to the second exemplaryembodiment of the present invention includes a gate electrode 221 formedon a certain substrate 210, a gate insulating layer 215 formed on thegate electrode 221, a primary active layer 224 a formed on the gateinsulating layer 215, source and drain electrodes 222 and 223, and asecondary active layer 224 b made of amorphous zinc oxide-basedsemiconductor and electrically connected with the source and drainelectrodes 222 and 223.

In this case, like the oxide TFT according to the first exemplaryembodiment, the oxide TFT according to the second exemplary embodimentof the present invention includes the secondary active layer 224 b for achannel by using the amorphous zinc oxide-based semiconductor, so thehigh mobility and constant current bias condition are satisfied anduniform characteristics are secured so as to be advantageouslyapplicable to a large-scale display device.

In particular, the oxide TFT according to the first exemplary embodimentof the present invention includes the secondary active layer 224 b madeof a-IGZO semiconductor containing heavy metal such as indium (In) andgallium (Ga) in the ZnO.

As for the oxide TFT according to the second exemplary embodiment of thepresent invention having such characteristics, a carrier density of thesecondary active layer 224 b can be adjusted by adjusting an oxygendensity within a reactive gas during sputtering, thus adjusting thedevice characteristics of the TFT.

The primary active layer 224 a according to the second exemplaryembodiment of the present invention may be made of amorphous zincoxide-based semiconductor including a-IGZO semiconductor like the uppersecondary active layer 224 b. After a primary semiconductor layer madeof oxide semiconductor is formed, wet etching is performed on the sourceand drain electrodes 222 and 223. Then, the source and drain electrodes222 and 223 are etched at a low selectivity, generating a taperedportion formed at the side. In this case, the tapered portion formed atthe side of the primary active layer 224 a acts as a contact regionbeing in contact with the upper secondary active layer 224 b, thuspreventing a disconnection of the secondary active layer 224 b due to astep of the source and drain electrodes 222 and 223.

As for the oxide TFT according to the first exemplary embodiment of thepresent invention, after the source and drain electrodes 222 and 223 areformed, a-IGZO oxide semiconductor is deposited to form the secondaryactive layer 224 b for a channel, thereby basically solving the problemof degeneration of the oxide semiconductor generated when the source anddrain electrodes 222 and 223 are etched.

In particular, in the oxide TFT according to the second exemplaryembodiment of the present invention, in order to improve theohmic-contact characteristics between the oxide semiconductor, namely,the secondary active layer 224 b and the source and drain electrodes 222and 223, the source and drain electrodes are formed as a dual-layer. Thesource and drain electrodes 222 and 223 include first source and drainelectrodes 222 a and 223 a in contact with the primary active layer 224a and second source and drain electrodes 222 b and 223 b formed on thefirst source and drain electrodes 222 a and 223 a and being in contactwith the secondary active layer 224 b.

In this case, the second source and drain electrodes 222 b and 223 b incontact with the secondary active layer 224 b may be made of metal suchas titanium (Ti) or a Ti alloy having a good bonding force with oxygenor indium tin oxide (ITO), molybdenum (Mo), and the like, having goodohmic-contact characteristics with the a-IGZO oxide semiconductor. Thiswill now be described in detail through a method of fabricating an oxideTFT.

FIGS. 5A and 5C are sectional views sequentially showing the fabricationprocess of the oxide TFT illustrated in FIG. 4.

As shown in FIG. 5A, the gate electrode 221 is formed on the substrate210 made of a transparent insulation material.

In this case, amorphous zinc oxide-based composite semiconductoremployed for the oxide TFT according to an exemplary embodiment of thepresent invention is available for a low temperature deposition, so thesubstrate 210 that can be applicable to a low temperature process suchas a plastic substrate, a soda lime glass, and the like, can be used. Inaddition, because the amorphous zinc oxide-based composite semiconductorhas amorphous characteristics, the large-scale display substrate 210 canbe used.

After a first conductive film is deposited on the entire surface of thesubstrate 210, it is selectively patterned through a photolithographyprocess (a first masking process) to form the gate electrode 221.

Here, the first conductive film may be made of a low-resistance opaqueconductive material such as aluminum (Al), an Al alloy, tungsten (W),copper (Cu), nickel (Ni), chromium (Cr), molybdenum (Mo), titanium (Ti),platinum (Pt), tantalum (Ta), and the like. Also, the first conductivefilm may be made of an opaque conductive material such as ITO, indiumzinc oxide (IZO), and the like, or may be formed with a multi-layeredstructure by stacking two or more conductive materials.

Next, as shown in FIG. 5B, the gate insulating layer 215 is formed onthe entire surface of the substrate 210 with the gate electrode 221formed thereon.

An amorphous zinc oxide-based semiconductor layer and second and thirdconductive films are formed on the entire surface of the substrate 210with the gate insulating layer 215 formed thereon, and then, the secondand third conductive films are selectively pattered through aphotolithography process (a second masking process) to form the sourceand drain electrodes 222 b and 223 b including the first source anddrain electrodes 222 a and 223 a and the second source and drainelectrodes 222 b and 223 b on the gate insulating layer 215.

At this time, the first active layer 224 a made of the amorphous zincoxide-based semiconductor is formed at the lower portion of the firstsource and drain electrodes 222 a and 223 a, and the oxide-basedsemiconductor is etched at a low selectivity during a wet etching of thesource and drain electrodes 222 and 223, generating a certain taperedportion formed at the side.

FIGS. 6A to 6C are sectional views showing a second masking process indetail according to the second exemplary embodiment of the presentinvention illustrated in FIG. 5B.

As shown in FIG. 6A, the gate insulating layer 215, which is formed ofan inorganic insulating layer such as silicon nitride film (SiNx) orsilicon oxide film (SiO₂) or a high dielectric oxide film such ashafnium (Hf) oxide or aluminum oxide, is formed on the entire surface ofthe substrate 210 with the gate electrode 221 formed thereon.

In this case, the gate insulating layer 215 may be formed throughchemical vapor deposition (CVD) or plasma enhanced chemical vapordeposition (PECVD).

And then, the amorphous zinc oxide-based semiconductor layer 220 made ofamorphous zinc oxide-based semiconductor and certain second and thirdconductive films 230 and 240 are formed on the entire surface of thesubstrate 210 with the gate insulating layer 215 formed thereon.

In this case, the amorphous zinc oxide-based semiconductor layer 220 maybe formed to have a thickness ranging from 10 Å to 2000 Å.

The second conductive film 230 may be used regardless of the metal kindsto form the first source and drain electrodes on the amorphous zincoxide-based semiconductor layer 220, and the third conductive film 240may be made of metal such as titanium or a titanium alloy having goodbonding force with oxygen or ITO, molybdenum, and the like, having goodohmic-contact characteristics with the a-IGZO oxide semiconductor. Inaddition, the source and drain electrodes may be formed to have amulti-layered structure including two or more layers.

As shown in FIG. 6B, a photosensitive film made of a photosensitivematerial such as photoresist is formed on the entire surface of thesubstrate 210 and a photosensitive film pattern 270 is formed throughthe second making process according to the second exemplary embodimentof the present invention.

Thereafter, as shown in FIG. 6C, the lower second and third conductivefilms are selectively removed by using the thusly formed photosensitivefilm pattern 270 as a mask to form the source and drain electrodes 222and 223 as a dual-layer including the first source and drain electrodes222 a and 223 a and the second source and drain electrodes 222 b and 223b at the upper portion of the gate insulating layer 215.

In this case, the source and drain electrodes 222 and 223 are etchedthrough a wet etching, and while the source and drain electrodes 222 and223 are being etched, the oxide-based semiconductor is etched at a lowselectivity compared with the second and third conductive films, formingthe primary active layer 224 a having a certain tapered portion at theside.

Here, the tapered portion formed at the side of the primary active layer224 a works as a contact area being in contact with the upper secondaryactive layer (not shown), thus preventing a disconnection of thesecondary active layer due to a step of the source and drain electrodes222 and 223.

As shown in FIG. 5C, amorphous zinc oxide-based semiconductor isdeposited on the entire surface of the substrate 210 with the source anddrain electrodes 222 and 223 of the dual-layer formed thereon to form acertain amorphous zinc oxide-based semiconductor layer, which is thenselectively patterned through a photolithography process (a thirdmasking process) to form the secondary active layer 224 b which iselectrically connected with the second source and drain electrodes 222 band 223 b.

In this case, the amorphous zinc oxide-based composite semiconductor,particularly, the a-IGZO semiconductor, may be formed through asputtering method by using a complex target of gallium oxide (Ga₂O₃),indium oxide (In₂O₃), zinc oxide (ZnO). Besides, it may be formedthrough a chemical deposition method such as the CVD, an atomic layerdeposition (ALD), and the like.

In addition, the a-IGZO semiconductor may be used to form an amorphouszinc oxide-based semiconductor layer by using composite oxide targetshaving atomic ratios of gallium, indium and zinc of 1:1:1, 2:2:1, 3:2:1,and 4:2:1, respectively.

Here, as for the oxide TFT according to the second exemplary embodimentof the present invention, a carrier density of the secondary activelayer 224 b can be adjusted by adjusting an oxygen density within areactive gas during sputtering to form the amorphous zinc oxide-basedsemiconductor layer, thus obtaining the device characteristics under anoxygen density 1% to 10% and under the thickness condition of 500 Å to1000 Å.

The secondary active layer 224 b according to the second exemplaryembodiment of the present invention is in contact with a contact area ofthe primary active layer 224 a having the tapered portion, thuspreventing disconnection of the secondary active layer 224 b due to astep of the source and drain electrodes 222 and 223. This will now bedescribed in detail with reference to the accompanying drawings.

FIG. 7 is a scanning electron microscope (SEM) photograph showing adisconnection of a semiconductor due to a step of source and drainelectrodes in an oxide TFT including only a secondary active layer, inwhich the channel area of the a-IGZO semiconductor TFT is enlarged to beshown.

With reference to FIG. 7, if source and drain electrodes with athickness of about 500 Å are formed and then an amorphous zincoxide-based semiconductor layer is deposited to form a secondary activelayer, the secondary active layer is disconnected at edge portions ofthe source and drain electrodes due to a step of the source and drainelectrodes.

FIG. 8 is an SEM photograph showing a primary active layer taperedthrough a wet etching of source and drain electrodes in the oxide TFTaccording to an exemplary embodiment of the present invention, and FIG.9 is an SEM photograph showing a state that the secondary active layeris in contact with the tapered primary active layer in the oxide TFTaccording to an exemplary embodiment of the present invention.

With reference to FIG. 8, it is noted that when the primary oxide-basedsemiconductor layer and a conductive film for the source and drainelectrodes are deposited and selectively patterned by using aphotosensitive film pattern as a mask, the primary active layer having acertain tapered portion is formed at a lower portion of the source anddrain electrodes.

Here, when a secondary oxide-based semiconductor layer is deposited toform a secondary active layer, as shown in FIG. 9, a sufficient contactarea is secured between the primary and secondary active layers,exhibiting good inter-bonding properties.

As so far described, the present invention can be applicable to adifferent display device fabricated by using TFTs, for example, an OLED(Organic Light Emitting Diode) display device in which OLEDs areconnected with driving transistors, as well as to the LCD device.

In addition, the present invention can be also applied for a transparentelectronic circuit or a flexible display as the amorphous zincoxide-based semiconductor material that has a high mobility and can beprocessed at a low temperature is applied as the active layer.

As the present invention may be embodied in several forms withoutdeparting from the spirit or essential characteristics thereof, itshould also be understood that the above-described embodiments are notlimited by any of the details of the foregoing description, unlessotherwise specified, but rather should be construed broadly within itsspirit and scope as defined in the appended claims, and therefore allchanges and modifications that fall within the metes and bounds of theclaims, or equivalents of such metes and bounds are therefore intendedto be embraced by the appended claims.

1. A method for fabricating a liquid crystal display (LCD) device, themethod comprising: forming a gate electrode on a substrate; forming agate insulating layer on the substrate; forming a primary active layerhaving a tapered portion to a side of a channel region of the primaryactive layer on the gate insulating layer, and forming source and drainelectrodes on the primary active layer; and forming a secondary activelayer made of amorphous zinc oxide-based semiconductor on the source anddrain electrodes and being in contact with the tapered portion of theprimary active layer, wherein the primary active layer is etched at alow selectivity during a wet etching of the source and drain electrodes,to have the tapered portion.
 2. The method of claim 1, wherein thesubstrate is formed as a glass substrate or a plastic substrate.
 3. Themethod of claim 1, wherein the primary active layer is made of amorphouszinc oxide (ZnO)-based semiconductor.
 4. The method of claim 1, whereinthe primary active layer is formed with a thickness ranging from 10 Å to2000 Å by using amorphous zinc oxide-based semiconductor.
 5. The methodof claim 1, wherein the source and drain electrodes comprise firstsource and drain electrodes formed on the primary active layer andsecond source and drain electrodes formed on the first source and drainelectrodes and electrically connected with the upper secondary activelayer.
 6. A liquid crystal display (LCD) device comprising: a gateelectrode formed on a substrate; a gate insulating layer formed on thegate electrode; a primary active layer formed on the gate insulatinglayer and having a tapered portion to a side of a channel region of theprimary active layer; source and drain electrodes formed on the primaryactive layer; and a secondary active layer formed on the source anddrain electrodes and made of amorphous zinc oxide-based semiconductor,wherein the secondary active layer is in contact with the taperedportion of the primary active layer to thus prevent disconnection of thesecondary active layer due to a step of the source and drain electrodes.7. The device of claim 6, wherein the primary active layer is made ofamorphous zinc oxide (ZnO)-based semiconductor.
 8. The device of claim6, wherein the primary active layer is formed with a thickness rangingfrom 10 Å to 2000 Å by using amorphous zinc oxide-based semiconductor.9. The device of claim 6, wherein the side of the primary active layeris tapered as the primary active layer is etched at a low selectivityduring a wet etching of the source and drain electrodes.